Crossing over produces recombinant chromosomes

Crossing over produces recombinant chromosomes - Crossing...

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Crossing over produces recombinant chromosomes, which combine genes inherited from each parent. ○ Crossing over begins very early in prophase I, as homologous chromosomes pair loosely along their lengths. ○ Each gene on one homolog is aligned precisely with the corresponding gene on the other homolog. ○ In a single crossover event, specific proteins orchestrate an exchange of the corresponding segments of two nonsister chromatids—one maternal and one paternal chromatid of a homologous pair. ○ In crossing over, homologous portions of two nonsister chromatids trade places. ○ As a result, individual chromosomes carry genes derived from two different parents. ○ For humans, this occurs an average of one to three times per chromosome pair. In humans and most other organisms, crossing over may be essential for the lining up of homologous chromosomes during metaphase I. ○ Chiasmata hold homologs together as the spindle forms in meiosis I.
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Unformatted text preview: Crossing over, by combining DNA inherited from two parents into a single chromosome, is an important source of genetic variation. At metaphase II, nonidentical sister chromatids sort independently from one another, further increasing the number of genetic types of daughter cells that are formed by meiosis. The random nature of fertilization adds to the genetic variation arising from meiosis. Any sperm can fuse with any egg. ○ The ovum is one of more than 8 million possible chromosome combinations. ○ The successful sperm is one of more than 8 million possibilities. ○ The resulting zygote could contain any one of more than 70 trillion possible combinations of chromosomes. ○ Crossing over adds even more variation to this. Each zygote has a unique genetic identity. All three mechanisms reshuffle the various genes carried by individual members of a population....
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This note was uploaded on 01/04/2012 for the course BSC BSC1005 taught by Professor Orlando,rebecca during the Fall '10 term at Broward College.

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